The invention relates to a multi-beam optical unit which is used for a high-speed copy machine and forms an image by simultaneously emitting a plurality of beams.
An image forming apparatus using a laser optical system is considered to improve a speed of forming an image on a photo conductor by having a plurality of laser beam emitting sources for printing at a high speed and simultaneously guiding the laser beams emitted from these laser beam emitting sources onto the photo conductor. It is also considered to form a color image on a drum by having laser beam emitting sources corresponding to respective colors for forming a color image and emitting from these beam emitting sources to form an image of each color.
As disclosed in Japanese Patent Laid-Open Publication No. 9-189872, the image forming device having the plurality of laser beams is considered to have a common optical member such as a pair of polygon mirrors and a f.theta. lens for the multiple beams in order to make the device compact. In order to enter the multiple beams into one pair of polygon mirrors, it is necessary to make the intervals among the multiple beams small to synthesize into one beam in appearance.
Therefore, the multiple beams are required to be synthesized by a half mirror before entering into the polygon mirrors. For example, where two beams are synthesized into one, one of the beams is passed through the half mirror, while the other is reflected by the half mirror. Here, where the laser beam of a conventional optical device is passed through the half mirror, its power is attenuated, causing a difference in beam power between the beam reflected by the half mirror and the one passed through the half mirror even if the beam sources having the same power are used.
Accordingly, Japanese Patent Laid-Open Publication No. 9-189872 described above has the optical member disposed so to allow the respective beams to pass through the half mirror in a minimum quantity with the power loss of the beam passing through the half mirror taken into account. The number of times to pass through the half mirror is limited to zero or one time.
But, when the number of times to pass through the half mirror in order to compose the multiple beams into one is decreased, the number of times to reflect the beams by the half mirror is increased. When the number of times to reflect the beams by the half mirror is increased, influences tend to be caused due to an error in mounting the half mirror and accuracy of a housing. If the half mirror were displaced, both the incident beam and the outgoing beam are influenced by the "displacement", and the influence becomes two times larger than the actual displacement. Besides, even if the displacement at the half mirror is very small, it becomes very large on the photo conductor drum field where the beam finally reaches because there is some distance from the half mirror to the field on the photo conductor drum. Furthermore, this displacement is multiplied with the increase in the number of reflection times. Thus, the displacement between the beam passing through the half mirror and the beam reflected by the half mirror is increased, and the positions of the multiple beams on the field are largely dispersed, resulting in a disadvantage that the image is degraded.